This invention relates to a process for producing a photographic master batch and a process for producing photographic resin coated paper. (2) Description of the Prior Art
Photographic resin coated papers containing a photographic master batch in the resin layer are already known. For instance, U.S. Pat. No. 3,501,298 discloses a photographic resin coated paper made by coating both sides of the support with a resin in which the resin layer on the side applied with a photographic emulsion contains a titanium dioxide pigment, a blue pigment and a fluorescent brightener.
The known processes for producing such photographic resin coated paper, however, are attended by some serious problems. For example, when a resin composition, especially a polyethylene resin composition containing a titanium dioxide pigment is melt extruded in the form of a film from a slit die, the needle- or icicle-like deposits or stains tend to form at the tip of the die lip in a short time or extrusion operation (such deposits or stains being hereinafter referred to simply as die lip stains), and to make the matter worse, such die lip stains tend to grow up rapidly as the melt extrusion is continued.
The die lip stains formed in the course of melt extrusion coating cause the vertical streaks or irregular coating weight due to non-uniform coating rate on the surface of the produced photogrpahic resin coated paper. In some cases, such stains stick to the film to form a contaminant in the coating. Thus, if such stains are left unremoved, the produced resin coated paper is badly impaired in its surface quality and can not be used as a photographic support paper which requires high surface quality.
Also, for perfectly removing the die lip stains in the extruder, there has been no alternative but to clean the die lip by stopping the production line, and a great deal of labor and time has been required for such cleaning. Thus, a prompt solution to this problem has been keenly desired.
Microgrits present another difficult problem in the production of photographic resin coated paper. Microgrits mean here the minute foreign matter or minute irregular grains or particles which come out in the coated resin surface of a photographic resin coated paper made by coating at least one side of a paper or synthetic paper base with a resin composition.
Various causes have been considered for the formation of such microgrits. In the case of the microgrits which are seen on the photographic resin coated paper produced by melt extrusion coating at least one side of a paper or synthetic paper base with a photographic resin by a melt extruder, the following causes are considered: (1) the resin used is of the type which tends to form gels; (2) when the molten resin is extruded into a film from a melt extruder through a die, the proper temperature is not maintained, causing a non-uniform flow of resin; (3) the screen set on the braker plate in the melt extruder is smudged; (4) the barrel liner in the melt extruder is cracked; (5) mixing of the resin composition in the melt extruder is insufficient. These problems may not be so difficult to solve for a skilled engineer.
The most difficult case with the microgrits is when such microgrits are generated on a photographic resin coated paper in which the paper or synthetic paper base is coated with a resin composition consisting of at least a resin and a pigment.
As means for incorporating a pigment in the resin layer, usually a so-called master batch is first prepared by containing a desired pigment in a resin to a certain predetermined concentration and then the batch is diluted with a diluting resin to provide a desired mixing ratio, or a so-called compound is prepared in which a proper pigment is contained in a resin in a desired ratio.
However, when such master batch or compound is prepared by melting and mixing a resin and a pigment by using an ordinary melt mixer, especially a Banbury mixer, kneader or such, the relatively coarse pigment particles tend to be dispersed in the form as they are, without being broken into the fine particles in the resin, allowing the existence of the undispersed pigment particles in the master batch or compound. This gives rise to the microgrits on the resin surface of the photographic resin coated paper in which at least one side of the paper or synthetic paper base is coated with a resin composition consisting of at least a resin and a pigment and produced by using said master batch or compound.
The existence of microgrits in the resin coated paper used as a photographic support causes serious photographic trouble. For instance, when the figure of a person is photographed on a printing paper using a resin coated paper having microgrits as a photographic support, said microgrits might come out at a crucial part such as the face of the photographed person, thus badly spoiling the photograph.